In the semiconductor industry the dominant light in photolithograph process is the 193 nm excimer laser that operates in the DUV (deep ultraviolet) region. Calcium fluoride (CaF) crystals and high purity fused silica (for example, HPFS®, Corning Incorporated) are the most important optical materials used in the optical systems operated in the DUV spectral region. For example, an objective or projection system may comprise many CaF2 and SiO2 lens elements with multilayer antireflection (AR) coatings. These lens elements need to be precisely mounted onto stainless steel mounts and assembled together in a specific sequence to form an optical system. In practice, UV-curable adhesives have been extensively used to secure a lens element onto its corresponding stainless steel mount. In the mounting process, an adhesive polymer compound is applied between the lens element and the metal mount. Solidification of the polymer compound by UV irradiation, for example, 365 nm UV light, bonds the lens element to the metal mount. However, it is known that the scatter of 193 nm light in an optical system may degrade the UV-curable adhesive and eliminate its long term stability or lifetime, thus resulting in miss alignment of the optical system over time. There are two possible approaches to extend stability of the UV-curable adhesive. These are:
(1) reducing the 193 nm scatter light of an optical system, and
(2) adding a selected protective coating to the optical element to prevent 193 nm scatter light damage at the adhesive-optical element boundary layer so that the adhesive is not degraded by 193 nm radiation.
The general idea of the protective coating is to insert a dielectric film between a lens element and its surrounding UV-curable adhesive. The dielectric film transmits the UV curing light at 365 nm and blocks 193 nm scatter light. The protective coating approach has been realized by means of physical and chemical deposited oxide films as described in, for example, in U.S. Pat. No. 6,097,536 (the '536 patent) a protective layer of Ta2O5, TiO2, HfO2 is deposited by vapor deposition of the material), and in U.S. Pat. No. 7,081,278 (the '278 patent), chemical deposition of metal oxide films such as SiO2, Al2O3, ZrO2, HfO2, Ta2O5, Nb2O5, and TiO2 using a sol-gel type process in which an organo-oxy metallic compound is used as chemical precursor followed by hydrolysis and condensation to form a metal oxide film after drying).
Other, different protective solutions involving new bonding materials and processes have also been under investigation. For example, Precision Photonics Corporation (www.precisionphotonics.com) recently claimed that a chemically activated direct bonding (CADB™) technology has been developed resulting in epoxy-free optical paths that are perfectly transparent with negligible scattering and absorptive losses at the bonding interfaces. The CADB technology is claimed to offer bond strengths often times equal to the strength of the bulk materials being bonded. However, the CADB™ technology is only good for bonding optically smoothed and flatted glass surfaces, but not for glass-metal contacts or rough surfaces.
Directly and indirectly protective coatings have also been applied to fluoride optics operating at 193 nm for reasons other than protecting UV-curable adhesives from degradation. U.S. Pat. No. 7,242,843 (the '843 patent) describes the use of a single layer of dense F—SiO2 (a fluorine doped silica) coating as a directly protective coating to prevent fluorine depletion in CaF2 optics under 193 nm light irradiation, leading to a prolonged lifetime of the F—SiO2 protected CaF2 optics. In US 2008/0204862, F—SiO2 layers are inserted between fluoride coating stacks on the surface of the optic through which light passes as indirectly protective coatings (of the layers below the F—SiO2 coating), and enable plasma smoothing and densification of fluoride coating layers and stacks without introducing additional absorption.
Thus, while advances have been made in protecting the UV-curable adhesives used for the bonding of optical elements to, for example, holders, there is a need for further improvement regarding such protection. The present disclosure present a novel method for protecting the adhesives used to bond optical elements used in DUV lasers to their holders so that the adhesive is not degraded in use.